1. Field of the Invention
The present invention relates to an ink jet recording method employing an ink jet system for photocopiers, printers, facsimile machines, etc.
2. Description of the Background Art
Among many printing systems, the ink jet recording system finds application over a wide range of printing fields since it can form images simply and inexpensively. In the ink jet recording technology, ink droplets that are formed through minute nozzles are attached to a recording medium such as paper according to image information using a pressure on-demand system, a charge control system, etc. Such an ink jet recording technology is suitably applied to image forming apparatuses such as printers, facsimile machines, and photocopiers. Since images can be formed by directly attaching ink to a recording medium in the ink jet recording technology, recording can be performed by a relatively simple device structure in comparison with indirect recording technologies such as electrophotography using an image bearing member.
However, printing performance and in particular printing speed has improved in the printing industry in recent years. Consequently, the typical aqueous ink or oil-based ink ejected onto the recording medium does not dry in time. To solve such a problem, using ultraviolet curable ink which is dried (cured) instantly upon irradiation by light on any kind of recording medium is proposed.
A specific example of ultraviolet curable ink is a combination of a radical reactive compound (radical monomer) and a photoradical initiator. This system is characterized by an extremely quick reaction but has a problem in that radicals generated by irradiation by light are soon deactivated by oxygen in the air, thereby terminating the reaction prematurely. In such a case, radical monomers having a low molecular weight remain in the ink, which causes problems with regard to safety, etc.
On the other hand, a system using a combination of a cation reactive compound (cation monomer) and a photo-acid-generating agent is known to prevent inhibition by oxygen. In this combination, since a reactive strong acid is not inhibited by oxygen (although susceptible to moisture and a base), the reaction (post-curing) continues after irradiation by light so that the remaining monomer can be reduced. However, the presence of such a strong acid causes problems such as corrosion or degeneration of printed media.
Further, although there are only a few examples, a system using a combination of an anion reactive compound (anion monomer) and a photobase generator is also known. For example, unexamined Japanese patent application publication no, 2005-060520 (JP-2005-060520-A) describes a curable composition used as ink for ink jet printing that is cured by adding an overt and/or potential base to an alicyclic epoxy monomer having a specific structure.
In such a system, similar to the cation system, a reactive base is not deactivated soon, so the reaction continues after irradiation by light. In addition, a weak base can be used, thereby causing no problem of corrosion or degeneration of printed media. However, with this system increasing the printing speed is difficult because the reaction speed is slow.
Moreover, if ink of the system described above is used as an ultraviolet curable ink, in particular an ink having a coloring agent, the curability thereof deteriorates and curing does not proceed sufficiently by initial irradiation by light.
Furthermore, in general, if an ultraviolet curable ink for ink jet printing is used for a non-permeable media in particular, the ink discharged is cured before permeation and leveling, so that ink dots tends to swell, resulting in formation of a rough surface. This leads to a problem of non-uniform gloss between the background portion and printed portion, which gives a sense of discomfort. Consequently, a good-quality image is difficult to obtain.
On the other hand, there is a known technology in which an ultraviolet curable clear liquid is applied to a recording medium as an undercoating where a colored ink is discharged, thereby preventing ooze and mixing between droplets. For example, JP-2008-248070-A describes usage of a colored liquid containing a cation polymerizable compound and a liquid for an underlying layer containing a radical polymerizable compound. JP-2010-076138-A describes usage of a colored liquid and a liquid for an underlying layer both of which contain a cation polymerizable compound. In addition, JP-2008-023980-A describes printing images after half-curing an undercoating.
In this method, quality images are formed on recording media by preventing surface deterioration of a liquid-applied surface that may occur in the process of applying a liquid for the underlying layer to the recording media. That is, the underlying layer is half-cured before the ink is discharged to prevent excessive spreading of ink droplets. However, satisfying the curing conditions that create this half-cured state is difficult, with the result that the underlying layer is not stably half-cured but completely or barely cured. As a consequence, the ink droplets do not uniformly spread.